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Chapter 341

Amebiasis denotes the disease caused solely by Entamoeba histolytica, although there are two other morphologically identical Entamoeba species that can also infect humans—Entamoeba dispar and Entamoeba moshkovskii. E dispar is about 10 times more prevalent than E histolytica in most endemic areas for amebiasis.1 Recent reports suggest that infection of E moshkovskii, which was once considered a free-living amoeba, is also common in some parts of the world such as Bangladesh, India, and Australia. Most E moshkovskii infections occur concomitantly with E dispar or E histolytica infections. Neither E dispar nor E moshkovskii infections are associated with disease.2 Free-living amoebic infections are discussed in Chapter 348.

Epidemiology

Amebiasis occurs worldwide but is much more common in developing nations. This is a result of contaminated water or food, leading to fecal-oral spread of the cyst. The World Health Organization estimates that E histolytica is second only to malaria as a protozoan cause of death. For example, the 1988 Mexican national serosurvey demonstrated serological evidence of E histolytica infection in 8.4% of the population.1 Nearly half of the children surveyed in a refugee camp in Dhaka, Bangladesh, had evidence of infection by age 5. Amebic dysentery is most common in grade-school children.3 Amebic liver abscess is 10 times more common in men than in women, being most common in men between the ages of 20 and 50 years.1,4 This male sex predominance is not observed in children. Steroid treatment and pregnancy appear to increase susceptibility to life-threatening infection. In the United States, most infections occur in immigrants or in those who travel to developing countries. Amebic liver abscess may present clinically with symptoms more than 6 months after travel to an endemic area. Residents of institutions for the mentally retarded and HIV-infected individuals are also at greater risk of E histolytica infection.

Pathophysiology

E histolytica is a protozoan with an invasive, motile trophozoite and infectious cyst stages that is responsible for person to person transmission of infection. The trophozoite varies in diameter from approximately 10 to 60 μm, has a clear ectoplasm, and a single nucleus. The cyst averages 12 mm in diameter and has 1–4 nuclei (Figs. 341-1 and 341-2). Humans are the only reservoir for E. Histolytica. Cysts that are passed in the feces of infected individuals survive in a moist environment for months. Following their ingestion in contaminated food or water the cysts travel to the small intestine, the multinucleated metacystic amoeba is activated and emerges through a hole in the cyst wall, and immediately after excysting it undergoes division into eight uninucleate trophozoites. These organisms do not colonize the small intestine but are usually carried to the cecum where they become established.

Figure 341-1.

Trophozoite of E. histolytica/E. dispar in direct ...

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